Perhaps the biggest problem with waste laptop batteries is that their usage history is unknown. It is important to run a series of tests to ensure that each cell is safe and useful. I have developed a production-line approach, and mark each cell with a “serial number” (YYMMDD## format) as I extract them from battery packs. This lets me record a range of data for each cell across each testing stage.
I read quite a lot of good ideas and put together the following test cycle:
- When I open a battery and extract the cells, I measure the “voltage at extraction”. Li-Ion cells have a normal voltage range of 2.7 – 4.2 V (extremes), but many waste laptop batteries have been sitting on shelves or in boxes for a long time. It is quite common to find cells that are significantly under-voltage, and some are even at 0 V.
- Any cells that are below 3 V get a careful “restoration” charge at just 50 mA to try and recover voltage. I use the NiMH setting on my smart charger for this step, and it is always supervised. Charging an under-voltage Li-Ion cell can cause shunts to form between the electrodes, creating an internal short-circuit (very bad). I understand that using a very low current helps prevent this.
- Cells are charged at 1.0 A on the LiPo setting of my hobby (RC planes) smart chargers, up to 4.2 V. Any cells that heat up while charging are instantly disqualified from further testing. I have not yet properly quantified “heat up”, but it is very obvious (they get hot to the touch). I record the date/time each cell finishes charging.
- One of my chargers gives an internal resistance reading. I am recording this for the cells that get charged on that device.
- The next major test is for self-discharge. I leave the cells sitting on the bench (in egg cartons) and measure their voltage after 24 hours, after 7 days, and after 14 days. I need cells which can hold most of their charge over this 2 week period.
- The final (and perhaps most important) thing to test is capacity. I want a standard test for this, and so typically give the cells a top-up charge to let them all start from “full”. I discharge them at 1 A on my smart charger, which logs how many mAh it draws from the cell. The cut-off is set to 3.2 V. I record the capacity. 1 A is a higher discharge current than most laptop cells would experience, but it is a nice round number and lets the discharge test be a bit quicker than if I used a lower current. It also serves as an upper-bound for the design of my home battery.
For improved safety, these tests are always conducted outside on a compressed fibre-cement sheet. Cells are never left charging while I am away from the house.